The present invention pertains to fuel cells, and more particularly to a direct methanol fuel cell including a water recovery and recirculation system and a method of fabricating the system, in which water is collected and recirculated during the process of generating electrical energy.
Fuel cells in general, are xe2x80x9cbattery replacementsxe2x80x9d, and like batteries, produce electricity through an electrochemical process without combustion. The electrochemical process utilized provides for the combining of protons with oxygen from air or as a pure gas. The process is accomplished utilizing a proton exchange membrane (PEM) or proton conducting membrane sandwiched between two electrodes, namely an anode and a cathode. Fuel cells, as known, are a durable provider of electricity. Hydrogen is typically used as the fuel for producing the electricity and can be processed from methanol, natural gas, petroleum, or stored as pure hydrogen. Direct methanol fuel cells (DMFCs) utilize methanol, in a gaseous or liquid form as fuel, thus eliminating the need for expensive reforming operations. DMFCs provide for a simpler PEM cell system, lower weight, streamlined production, and thus lower costs.
In a standard DMFC, a dilute aqueous solution of methanol is fed as the fuel on the anode side (first electrode), and the cathode side (second electrode) is exposed to forced or ambient air (or O2). A proton conducting membrane, such as a Nafion(copyright) type proton conducting membrane, typically separates the anode and the cathode sides. Several of these fuel cells can be electrically connected in series or parallel depending on voltage or power requirements.
Typically, DMFC designs are large stacks with forced airflow at elevated temperatures. Smaller air breathing DMFC designs are more complicated. In conventional PEM fuel cells, stack connections are made between the fuel cell assemblies with conductive plates, machined with channels or grooves for gas distribution. A typical conventional fuel cell is comprised of an anode (H2 or methanol side) current collector, anode backing, membrane electrode assembly (MEA) (anode/ion conducting membrane/cathode), cathode backing, and cathode current collector. Each fuel cell is capable of producing approx. 1.0 V. To obtain higher voltages, fuel cells are typically stacked in series (bi-polar mannerxe2x80x94positive to negative) one on top of another. Conventional fuel cells can also be stacked in parallel (positive to positive) to obtain higher current, but generally larger fuel cells are simply used instead.
During operation of a direct methanol fuel cell, a dilute aqueous methanol (usually 3-4 wtwt % methanol) solution is used as the fuel on the anode side. If the methanol concentration is too high, then there is a methanol crossover problem that will greatly reduce the efficiency of the fuel cell. If the methanol concentration is too low then there will not be enough fuel on the anode side for the fuel cell reaction to take place. Current large DMFC stack designs utilize forced airflow. The smaller air breathing DMFC designs are difficult to accomplish because of the complexity in miniaturizing the system for portable applications.
Carrying the fuel in the form of a very dilute methanol mixture would require carrying a large quantity of fuel which is not practical for the design of a miniature power source for portable applications. Miniaturizing the DMFC system requires having on hand separate sources of methanol and water and mixing them in-situ for the fuel cell reaction. To aid in supplying methanol and water to the anode, it would be beneficial to recirculate the aqueous fuel mixture after the fuel cell reaction, and recycle the water generated at the cathode in the fuel cell reaction, as well as the water arriving at the cathode via diffusion and electro-osmotic drag.
Accordingly, it is a purpose of the present invention to provide for a direct methanol fuel cell system design in which a water management system is integrated into a miniaturized device.
It is a purpose of the present invention to provide for a direct methanol fuel cell including a water management system, comprised of microchannels, cavities, hydrophobic/hydrophilic treatments, and microfluidics technology for fuel-bearing fluid mixing, pumping and recirculation of water by-product production.
It is still a further purpose of the present invention to provide for a direct methanol fuel cell including a water management system in which all of the system components are either embedded inside a base portion, or positioned adjacent a base portion, such as a ceramic base portion.
It is yet a further purpose of the present invention to provide for method of fabricating a direct methanol fuel cell including a water management system, comprised of microchannels, cavities, chemical surface modifications, and microfluidics technology for fuel-bearing fluid mixing, pumping and recirculation of water by-product production.
The above problems and others are at least partially solved and the above purposes and others are realized in a fuel cell device and method of forming the fuel cell device including a base portion, formed of a singular body, and having a major surface, and a cap portion. At least one membrane electrode assembly (MEA) is formed on the major surface of the base portion. The membrane electrode assembly is in communication with a stream of forced air. The forced air is passed over the cathode through a flow field formed in the cap portion. Oxygen in the air is utilized by the MEA to generate electricity, and the water byproduct formed at the cathode, the water and methanol which diffused through the membrane, is carried away by the stream of forced air, thus exiting across the MEA as a combination of air, oxygen depleted air, or remaining oxygen, water, and possibly methanol. The stream of forced air subsequently enters an gas-liquid separator. The air exits the separator through a hydrophobic membrane, while the water is re-introduced into a recirculating channel defined in the base portion. The recirculating channel is spaced apart from the fluid supply channel for re-introducing by-product fluid/water, from the at least one membrane electrode assembly. A fluid supply channel is defined in the base portion and communicates with the at least one membrane electrode assembly for supplying a fuel-bearing fluid to the at least one membrane electrode assembly. The membrane electrode assembly including the base portion and the cap portion form a single fuel cell assembly.